Thermoplastics comprise a large family of polymers, most of which have a high molecular weight. Intermolecular forces are responsible for the association of the molecular chains, which allows thermoplastics to be heated and remolded. Thermoplastics become pliant and moldable at a temperature above their glass transition temperature but below their melting point, and the intermolecular forces reform after molding and upon cooling of the thermoplastic, resulting in the molded product having substantially the same physical properties as the material prior to molding.
Polycarbonates fall within the thermoplastic family and contain carbonate groups —O—(C═O)—O—. Polycarbonates find widespread use throughout industry due to their excellent strength and impact resistance. Additionally, polycarbonates may be readily machined, cold-formed, extruded, thermoformed and thermomolded.
Exposure of thermoplastics to light is known to induce changes to the polymer. In particular, the exposure of opaque, translucent and transparent polycarbonates to blue LED (light emitting diode) light is of interest for the manufacture of efficient illumination devices such as lamps and other types of lighting apparatuses. Transparent is defined as a light transmittance of at least 80% when tested in the form of a 3.2 mm thick test sample according to ASTM D1003-00 (2000) (hereby incorporated by reference). Translucent is defined as a light transmittance greater than or equal to 40% when tested in the form of a 2.5 mm thick test sample according to ASTM D1003-00 (2000). Opaque is defined as a light transmittance of 10% or greater when tested in the form of a 3.2 mm thick test sample according to ASTM D1003-00 (2000). The testing according to ASTM D1003-00 (2000) uses procedure A and CIE illuminant C and 2 degree observer on a CE7000A using an integrating sphere with 8°/diffuse geometry, specular component included, UV included, large lens, and large area view, with percentage transmittance value reported as Y (luminous transmittance) taken from the CIE 1931 tristimulus values XYZ.
Blue LED light having a peak intensity from about 400 nm to about 500 nm and an irradiance from about 3,500 W/m2 to about 120,000 W/m2 is of particular interest. Similarly, white LED light having a peak intensity from about 400 nm to about 500 nm and an irradiance less than 120,000 W/m2 is also of interest.
Opaque and translucent polycarbonate may be formed, for example, using titanium dioxide compounded with the polycarbonate formulation. Furthermore, remote phosphors, also known as “luminescent conversion materials”, can be compounded into the polycarbonate. Examples of luminescent conversion materials include yttrium aluminum garnet (YAG) doped with rare earth elements, terbium aluminum garnet doped with rare earth elements, silicate (BOSE) doped with rare earth elements; nitrido silicates doped with rare earth elements; nitride orthosilicate doped with rare earth elements, and oxonitridoaluminosilicates doped with rare earth elements.
Translucent polycarbonates are formed using scattering agents such as light diffusers. The light diffusers often take the form of light diffusing particles and are used in the manufacture of articles that have good luminance. Such articles provide a high level of transmission of incident light (such as natural light through a window or skylight, or artificial light) with a minimum light loss by reflectance or scattering, where it is not desirable to either see the light source or other objects on the other side of the article.
An article e.g., a sheet having a high degree of hiding power (i.e., luminance) allows a significant amount of light through, but is sufficiently diffusive so that a light source or image is not discernible through the panel. Light diffusers can be (meth)acrylic-based and include poly(alkyl acrylate)s and poly(alkyl methacrylate)s. Examples include poly(alkylmethacrylates), specifically poly(methyl methacrylate) (PMMA). Poly(tetrafluoroethylene) (PTFE) can also be used. Light diffusers also include silicones such as poly(alkylsilsesquioxanes), for example poly(alkylsilsesquioxane)s such as the poly(methylsilsesquioxane) available under the trade name TOSPEARL® from Momentive Performance Materials Inc. The alkyl groups in the poly(alkyl acrylate)s, poly(alkylmethacrylate)s and poly(alkylsilsesquioxane)s can contain one to about twelve carbon atoms. Light diffusers can also be cross-linked. For example, PMMA can be crosslinked with another copolymer such as polystyrene or ethylene glycol dimethacrylate. In a specific embodiment, the polycarbonate composition comprises a light diffusing crosslinkedpoly(methyl methacrylate), poly(tetrafluoroethylene), poly(methylsilsesquioxane), or a combination comprising at least one of the foregoing.
Light diffusers also include certain inorganic materials, such as materials containing antimony, titanium, barium, and zinc, for example the oxides or sulfides of antimony, titanium, barium and zinc, or a combination containing at least one of the forgoing. As the diffusing effect is dependent on the interfacial area between polymer matrix and the light diffuser, in particular the light diffusing particles, the particle size of the diffusers can be less than or equal to 10 micrometers (μm). For example, the particle size of poly(alkylsilsesquioxane)s such as poly(methylsilsesquioxane) can be 1.6 μm to 2.0 μm, and the particle size of crosslinked PMMA can be 3 μm to 6 μm. Light diffusing particles can be present in the polycarbonate composition in an amount of 0 to 1.5%, specifically 0.001 to 1.5%, more specifically 0.2% to about 0.8% by weight based on the total weight of the composition. For example, poly(alkylsilsesquioxane)s can be present in an amount of 0 to 1.5 wt. % based on the total weight of the composition, and crosslinked PMMA can be present in an amount of 0 to 1.5 wt. % based on the total weight of the composition.
While the physical characteristics of strength and impact resistance make polycarbonates desirable for use as covers and lenses in LED lighting, exposure of the polycarbonate to the blue light of the LEDs (as well as the light of organic LEDs) causes degradation of the polycarbonate in the form of discoloration. For example, transparent polycarbonate is known to turn yellow, even darkening to brown, with exposure to blue light. This degradation of the transparent polycarbonate is unacceptable because the yellowed polycarbonate absorbs the light thereby reducing the efficiency of the lamp. Furthermore, the yellowing changes the color of the light emanating from the lamp, which is also unacceptable. In addition, the transparent polycarbonate is also subject to elevated temperatures when it comprises part of an LED lamp. The elevated temperatures likely play a role in the yellowing of the transparent polycarbonate.
There is clearly a need for a method to determine the degradation of thermoplastic formulations, in particular, the rate of discoloration of opaque, translucent and transparent polycarbonate formulations when exposed to light, and thereby be able to evaluate and compare the different formulations as to their suitability for use in LED lamps. It is desirable that the method provide an accelerated test of the thermoplastic which is not too slow so as to be impractical, and not too fast, so as to destroy the samples before meaningful comparisons can be made between formulations.